Why isn't changing the material of an object visibly working?
Because you're not in LookDev or Rendered mode (https://docs.blender.org/manual/en/latest/editors/3dview/controls/shading.html). 

As a side note, maybe you should start out with a basic tutorial, before you hop into your own projects. It may seem discouraging to actually having to learn something before one can produce results, but so is pretty much every skill that is not trivial.